pragma solidity ^0.6.12;

// SPDX-License-Identifier: SEE LICENSE IN LICENSE

/*
  * This is a MOCK token used for DEVELOPMENT PURPOSES ONLY.
  * It allows anyone to freely mint tokens with themselves as beneficiaries
  * Code is based on the ERC20Token contract from Bancor's contracts-solidity repository
  *
  * @dev ERC20 Standard Token implementation
*/

contract GiftableToken { //is IERC20Token, Utils {
    //using SafeMath for uint256;

    string public name;
    string public symbol;
    uint8 public decimals;
    uint256 public totalSupply;
    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;

    /**
      * @dev triggered when tokens are transferred between wallets
      *
      * @param _from    source address
      * @param _to      target address
      * @param _value   transfer amount
    */
    event Transfer(address indexed _from, address indexed _to, uint256 _value);

    /**
      * @dev triggered when a wallet allows another wallet to transfer tokens from on its behalf
      *
      * @param _owner   wallet that approves the allowance
      * @param _spender wallet that receives the allowance
      * @param _value   allowance amount
    */
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);

    event ChangedSupply(uint256 _oldTotal, uint256 _newTotal);

    /**
      * @dev initializes a new ERC20Token instance
      *
      * @param _name        token name
      * @param _symbol      token symbol
      * @param _decimals    decimal points, for display purposes
      * @param _initialSupply total supply of token units
    */
    constructor(string memory _name, string memory _symbol, uint8 _decimals, uint256 _initialSupply) public {
        // validate input
        require(bytes(_name).length > 0, "ERR_INVALID_NAME");
        require(bytes(_symbol).length > 0, "ERR_INVALID_SYMBOL");

        name = _name;
        symbol = _symbol;
        decimals = _decimals;
        totalSupply = _initialSupply;
        balanceOf[msg.sender] = _initialSupply;
    }

    // validates an address - currently only checks that it isn't null
    modifier validAddress(address _address) {
        _validAddress(_address);
        _;
    }

    // error message binary size optimization
    function _validAddress(address _address) internal pure {
        require(_address != address(0), "ERR_INVALID_ADDRESS");
    }


    function gift(address _to, uint256 _value) 
	public
	virtual
	validAddress(_to)
	returns (bool)
        {
	totalSupply = totalSupply + _value;
	uint256 oldSupply = balanceOf[_to];
	balanceOf[_to] = balanceOf[_to] + _value;
	emit ChangedSupply(oldSupply, balanceOf[_to]);
	emit Transfer(address(0x00), _to, _value);
	return true;
    }

    /**
      * @dev transfers tokens to a given address
      * throws on any error rather then return a false flag to minimize user errors
      *
      * @param _to      target address
      * @param _value   transfer amount
      *
      * @return true if the transfer was successful, false if it wasn't
    */
    function transfer(address _to, uint256 _value)
        public
        virtual
        validAddress(_to)
        returns (bool)
    {
        balanceOf[msg.sender] = balanceOf[msg.sender] - _value;
        balanceOf[_to] = balanceOf[_to] + _value;
        emit Transfer(msg.sender, _to, _value);
        return true;
    }

    /**
      * @dev transfers tokens to a given address on behalf of another address
      * throws on any error rather then return a false flag to minimize user errors
      *
      * @param _from    source address
      * @param _to      target address
      * @param _value   transfer amount
      *
      * @return true if the transfer was successful, false if it wasn't
    */
    function transferFrom(address _from, address _to, uint256 _value)
        public
        virtual
        validAddress(_from)
        validAddress(_to)
        returns (bool)
    {
        allowance[_from][msg.sender] = allowance[_from][msg.sender] - _value;
        balanceOf[_from] = balanceOf[_from] - _value;
        balanceOf[_to] = balanceOf[_to] + _value;
        emit Transfer(_from, _to, _value);
        return true;
    }

    /**
      * @dev allows another account/contract to transfers tokens on behalf of the caller
      * throws on any error rather then return a false flag to minimize user errors
      *
      * also, to minimize the risk of the approve/transferFrom attack vector
      * (see https://docs.google.com/document/d/1YLPtQxZu1UAvO9cZ1O2RPXBbT0mooh4DYKjA_jp-RLM/), approve has to be called twice
      * in 2 separate transactions - once to change the allowance to 0 and secondly to change it to the new allowance value
      *
      * @param _spender approved address
      * @param _value   allowance amount
      *
      * @return true if the approval was successful, false if it wasn't
    */
    function approve(address _spender, uint256 _value)
        public
        virtual
        validAddress(_spender)
        returns (bool)
    {
        // if the allowance isn't 0, it can only be updated to 0 to prevent an allowance change immediately after withdrawal
        require(_value == 0 || allowance[msg.sender][_spender] == 0, "ERR_INVALID_AMOUNT");

        allowance[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }
}